1. Field of the Invention
The present invention relates to an injection molding method of an injection molding machine for molding by injecting and charging a measured resin into a mold by means of a pushing-out member.
2. Description of the Relevant Art
Until now, an injection molding machine, for example, an in-line screw type of the injection molding machine is known in the official gazette of Japanese Patent Application Laid-Open No. 63 (1988)--209918, and an injecting molding method using this type of the injection molding machine is known in the official gazette of Japanese Patent Application Laying-Open No. 3 (1991)--221427.
According to this injection molding method, at first, a resin is measured in a measuring process. In the measuring process, a molding material supplied by a hopper disposed at a rear portion of a barrel cylinder is platicized and molten by a rotation of a screw, and is stored (measured) before the screw. On the other hand, after the measuring process is finished, it is shifted to an injecting process. In the injecting process, the screw is advanced based on a speed control, and a measured resin is injected and charged into a mold, and also after the charging is finished, that is, when it reaches a speed-pressure switching position (V-P switching position), it is shifted to a pressure retaining process. Then, in the pressure retaining process, a predetermined pressure retaining force is given for a charged resin within the mold based on the pressure control.
However, in such a prior injection molding method, there are the following problems because it needs at least a measuring process, an injecting process and a pressure retaining process.
First, it includes the pressure retaining process associated with a pressure control. Therefore, it needs to reserve a cushion amount by leaving a resin before a screw after the charging of the resin to the mold is finished. Thereby a residue resin and a new resin which are different in a melting time are mixed in the resin charged in the mold before and after. Accordingly, this results in a variation and a quality reduction of a molding product.
Second, it includes the pressure retaining process. Therefore, it needs a pressure control valve, a control circuit and the like which are used to control a pressure retaining operation. Also, in a V-P switching position at which it is switched from the injecting process in which the speed control is carried out, to the pressure retaining process in which the pressure control is carried out, an overshoot associated with a feedback control is generated. Therefore, it needs a complicated control circuit for ensuring stability and smoothness of the control. As a result, the control system is, expensive and large.